Optical waveguide gratings are important elements in optical communication systems. Waveguide gratings comprise a sequence of refractive index perturbations located along a waveguide. A grating freely transmits some wavelengths but not others depending on the spacing between perturbations. The distances between the perturbations and the index of refraction of the supporting optical structure determine the grating's transmission wavelength. Gratings can be fabricated in optical fibers or in planar waveguides as integrated optical structures.
Gratings are generally narrowband devices that reflect a wavelength that is fixed at the time of manufacture. The reflected bandwidth can be somewhat broadened by varying the spacing between perturbations (“chirping”) but even chirped devices are fixed at a relatively narrow bandwidth. It is important for many applications that gratings be wavelength tuneable. In some applications, tuning is needed to achieve a specific desired transmission and/or reflection wavelength. In other applications, tuning can be used to switch the grating among a plurality of transmission and/or reflection wavelengths. Wavelength tuning has been accomplished by altering the grating temperature with electrical heaters. Typically a current is passed through a deposited metal film thus raising the temperature of the film by ohmic heating. Changing the temperature of a grating changes the grating's transmission and/or reflection wavelength by changing the index of refraction in the waveguide (optical fiber in the case of a fiber grating) and to a lessor extent, by changing the spacing between perturbations.
Unfortunately, existing electrically heated tunable gratings, as well as other electrically heated tuneable optical devices, exhibit a limited useful tuning range. This is because heat tuning becomes non-linear at higher currents. Nonlinear tuning is particularly problematic in devices such as linearly chirped Bragg gratings, where the distance between perturbations varies linearly along the length of the grating. At higher heater currents the distances between the grating lines begins to change non-linearly.
Accordingly, there is a need for a way to fabricate heat tunable optical devices such that they can exhibit linear tuning characteristics over a far greater tuning range than is currently available.